Impact of disequilibrium compaction and unloading on overpressure in the southern Junggar Foreland Basin, NW China

Abstract

Disequilibrium compaction caused by burial or tectonic compression and unloading are the common overpressure mechanisms in foreland basins. However, accurately identifying multiple overpressure origins in such basins is challenging, since it requires distinguishing impact of vertical and horizontal stresses on compaction and overpressure as well as identifying overpressure caused by unloading. This paper proposed a dual compaction factor method (DCFM) to address these issues. A basic DCFM schematic diagram was established through analyzing compaction behaviors before and after tectonic compression to distinguish overpressure derived from two types of disequilibrium compaction and overpressure induced by unloading. In DCFM, a compaction model for mudstone in the foreland basin, which was only controlled by burial, was established based on the relationship between mean deposition rate and compaction factor in weak tectonic compression zone (WTCZ). It was used to determine compaction factor, normal compaction trend (NCT), acoustic transit time, bulk density, and overburden pressure under this compaction condition, as well as to distinguish the impact of burial and tectonic compression on porosity and overpressure at strong tectonic compression zone (STCZ). The DCFM employs mean stress to revise the equivalent depth method (EDM). Pore pressures associated with compaction only derived from burial and derived from both burial and tectonic compression were estimated using conventional EDM and revised EDM, respectively. The total pore pressures were estimated using the Eaton method and were calibrated based on measured ones. Overpressure associated with unloading was determined by comparing total pore pressure with estimated pore pressure generated with disequilibrium compaction. This mode was further applied at two wells from the Sikeshu Sag in the southern Junggar Foreland Basin to reconstruct accurate overpressure profiles with multiple origins, which were consistent with measured pressures and matched well with geological characteristics. Disequilibrium compaction induced by tectonic compression and burial is responsible for overpressure at the Sikeshu Sag. Overpressure transfer is also one of the primary contributors to the excess pressure in the Jurassic and Cretaceous in the Gaoquan anticline at the Sikeshu Sag, with a maximum contribution of 38 %. Plays, consisting of sandstone reservoirs with both transferred overpressure and tectonic-induced overpressure as well as mudstone caprocks with overpressure due to disequilibrium compaction, are favorable for hydrocarbon accumulation. This study can quantify the impact of overpressures with multiple origins on hydrocarbon migration and sealing capacity of caprocks, as well as provide ideas for researching fluid evolution in sedimentary basins.